In Dix et al. U.S. Pat. No. 5,112,570 entitled Two Phase Pressure Drop Reduction BWR Assembly Design, the concept of partial length rods is disclosed. This concept may best be understood by first understanding the construction of a conventional fuel bundle with conventional fuel rods and thereafter setting forth in summary format the part length rod invention of the above referenced patent, which patent is incorporated by reference in this disclosure.
A conventional boiling water fuel bundle includes a lower tie plate for supporting a matrix of vertically upstanding fuel rods at their lower end and an upper tie plate for holding the same matrix of vertical upstanding sealed fuel rods at their upper end. The lower tie plate permits the inflow of liquid moderator (water) around the fuel rods, the upper tie plate permits the outflow of liquid moderator (water) and generated vapor moderator (steam) at the upper end of the fuel bundle. A channel surrounds the lower tie plate, surrounds the upper tie plate, and defines a confined flow path for the moderator between the upper and lower tie plates about the matrix of upstanding fuel rods. Fluid flow is thus confined between the upper and lower tie plates and isolated from surrounding liquid moderator in the so-called core by pass area immediate adjoined to the fuel channel on the outside of the fuel channel.
The fuel rods within the fuel bundle are flexible and unless otherwise restrained would come into contact with one another under the forces of flow induced vibration and so-called "creep"--a differential growth in the fuel rods resulting from their heated, pressurized and radioactive environment. This being the case, a system of fuel rod spacers is distributed from the top to the bottom of the fuel bundles. These spacers form a matrix of individual cells discretely surrounding each fuel rod at spaced apart elevations within the fuel bundle holding the flexible fuel rods in their designed side-by-side relationship.
During operation of the fuel bundle in a reactor core of a boiling water nuclear reactor, the fuel bundle can be dynamically described as having two regions of operation. These regions include a lower single phase region containing liquid moderator (water) and an upper two phase region containing liquid moderator (water) intermixed with increasing fractions of vapor moderator (steam).
In a boiling water nuclear reactor, the moderator serves two purposes. First, the moderator moderates fast neutrons generated by the reaction into slow or thermal neutrons necessary to continue the reaction.
Secondly, the dense water moderator turns to expanded saturated steam. Energy is extracted from the saturated steam by passing the steam through an engine, such as a steam turbine.
Having set forth the conventional construction and operation of a boiling water nuclear reactor fuel bundle, the concept of part length rods can now be discussed.
In the above reference Dix et al. U.S. Pat. No. 5,112,570, so-called part length rods were disclosed. In short, an invention is set forth in which "a plurality of fuel rods extending from (the) lower tie plate toward the upper tie plate , (the) part length rods terminating within the two phase region of the bundle before reaching the upper tie plate;". The invention makes the point that "at least two of the part length rods (are) separate from one another so as to define in at least two locations in (the) bundle spaced apart and separate vents commencing at the top of said partial length rods and extending to (the) upper tie plate." Specifically, "each of (the) spaced apart vents (is) immediately adjoined by adjacent full length fuel rods."
Usefulness of the invention is set forth. Specifically, an improved fuel to moderator ratio is created in the upper two phase region of the fuel bundle. More importantly, pressure drop reduction in the upper two phase region of the fuel bundle is set forth. This enables greater stability of the fuel bundle and a reactor core including fuel bundles against thermal hydraulic and nuclear, thermal hydraulic instabilities. Additionally, the fuel to moderator ratio is improved, especially in the cold operating state.
The reader will understand at this point that the disclosed part length rods relate to the required spacers in two ways. First, since the part length rods do not extend to the upper tie plate, it is the spacers that hold the part length rods vertically upright at their respective upper ends. Secondly, since the part length rods terminate before the upper tie plate, some of the spacers in the upper two phase region of the fuel bundle overlying the ends of the part length rods. As will hereinafter appear, it is these spacers that constitute an obstacle in the desired removal of the part length rods.
It is common to inspect fuel bundles, and especially the individual fuel rods of fuel bundles during the operational life times of the fuel bundles. Unfortunately, the very presence of the part length rods renders the inspection of the part length rods inherently difficult. A brief understanding of the constraints of such inspections can be helpful.
Inspections of fuel bundle parts are typically made during reactor outages. During such reactor outages, the power output of the reactor is lost. This loss of power outage carries with it a corresponding loss in revenue. Any delay prolonging the reactor outage can be costly--running into lost revenues of well over several hundred thousand dollars per hour!
Accordingly, provision must be made for rapid inspection of all parts of a fuel bundle, including the new part length rods.
Conventional inspection of fuel bundles is typically accomplished in a submerged environment within a so-called "holding pool." The fuel bundle removed from the reactor is placed upright within the holding pool. Thereafter, the channel surrounding the fuel rods and the upper tie plate holding the fuel rods are removed. In the case of the fuel bundle having nothing but full length rods, the individual fuel rods may thereafter be accessed at the exposed top of each fuel rod, individually removed, inspected and replaced. Access to the fuel rods occurs at the top of the fuel rods. Such access is a routine matter.
Unfortunately, part length rods present special problems.
First, the part length rods have a length from the lower tie plate that is less than the full length rods. For example, the typical full length fuel rod is in the order of 160 inches in length; the typical part length fuel rod is in the order of 120 inches in length. In order to reach the part length rod, one has to penetrate a matrix of full length fuel rods. No view of the engagement of the part length rod is possible. All engagement is essentially "blind."
Secondly, since the part length fuel rods do not extend to the upper tie plate, the part length fuel rods are held in their upright position by the spacers. Accordingly, any tool for the removal and replacement of the part length fuel rods can have a diameter dimension no greater than the diameter dimension of the fuel rods being inspected. It is required that the tool for the removal of the part length rods pass through the spacers overlying the part length rods.
Finally, it is required in some applications that the part length rods be fastened against vertical movement to the lower tie plate. This is done by screwing the part length rods into receiving threads on the lower tie plate. Accordingly, the disassembly of such part length rods is correspondingly rendered more complicated. The screw threads stick. The part length fuel rod must be forcibly rotated. Accordingly, there is a need for both a tip on the part length rod and corresponding tools to render the rapid replacement and removal of such part length rods during reactor outages reliable and fast.